Ronaldo,

some emailprovider sends a message to the sender when a message lands
in a spamfolder [thus sending a confirmation to spammers that the
address is live...]

Note that the FBL mechanism is based on a contract between the sending service and the receiving service - just any old spam source won't be getting reports (assuming the receiving service has been careful enough).

Too, the amount of information reported to the sending service may vary - some receiving services don't include the email address that reported the message as spam.
https://en.wikipedia.org/wiki/Feedback_loop_(email)

Shal